Backing sheet coated with catalyst and self curing interpolymer adhesive

ABSTRACT

Pressure-sensitive resin solutions containing an acid-epoxy self-curing interpolymer and 1,3-bis(dimethylamino)-2-hydroxypropane. The solutions exhibit improved viscosity stability and provide pressure-sensitive resin films with cohesive strength and low release values from silicone release paper.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a division of Application Ser. No. 333,916, filedFeb. 20, 1973 now U.S. Pat. No. 3,893,982, issued July 8, 1975.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to organic solvent solutions containingacid-epoxy pressure-sensitive adhesive resins capable of self-cure forincreased cohesive strength, the self-curing reaction being catalyzed bya tertiary amine. In another aspect, the invention relates to a processof manufacture of articles containing films of pressure-sensitive resin.

2. Description of the Prior Art

Pressure-sensitive resins are used conventionally in the form of filmsto provide adhesive bonds between normally non-adhering superstrates andsubstrates. The films may be prepared by casting from organic solventsolutions and evaporating the solvent.

Constraints are placed on the molecular weight of the pressure-sensitiveresin by the viscosity of the solution necessary for ease of coating oncommericial coating machines and by the solution concentration requiredfor economical operation. In general, low solution viscosity and highsolution concentration are desired in the pressure-sensitive resinsolution, and, hence, a resin of relatively low molecular weight ispreferred for film casting.

Once a bond has been made by a pressure-sensitive film placed between asubstrate and a superstrate, the pressure-sensitive film may besubjected to stress generated by the weight of the superstrate or bydimensional change in the substrate or superstrate caused by thermalexpansion, stress relaxation, plasticizer migration or the like. Ingeneral, a high molecular weight and a high cohesive strength is desiredin the pressure-sensitive resin so that it may resist the stressesinduced in the adhesive film. The opposing requirements of low molecularweight resin for ease of coating and high molecular weight resin forload holding ability are conventionally reconciled by the use ofcrosslinkable resin systems which can be applied at low molecular weightand cured to a high molecular weight.

Pressure-sensitive adhesive resins containing epoxy and acid groups arewidely used because of their ability to self-crosslink on drying andheating to produce pressure-sensitive adhesive films with high cohesivestrength. However, the rate of curing of these systems is often too slowfor drying and curing conditions used in practice and a catalyst for theacid-epoxy reaction must then be added to the solution before coating.

Tertiary amines are widely accepted as catalysts for the acid-epoxyreaction. Generally, however, the curing rate with the majority oftertiary amines is still insufficient for the drying and curingconditions used in practice. One tertiary amine which has proved to beexceptionally effective for the crosslinking reaction istriethylenediamine. Triethylenediamine has the disadvantage, however, ofconferring very short pot life to the pressure-sensitive resin solution.In commercial use, pot life determines the useful coating life time ofcatalyzed batches of pressure-sensitive adhesive solutions. Catalyzedsolutions with short pot lives place severe limitations on theusefulness of the material, regardless of the properties of theadhesive. Furthermore, triethylene-diamine causes an interaction betweenthe pressure-sensitive adhesive and silicone release substrates,particularly if the release substrates are not substantially completelycured. This leads to undesirably high adhesion between the adhesive andrelease surface with the result that the release paper is difficult toremove and the adhesive properties of the pressure-sensitive adhesivefilm are impaired. There is, therefore, a need in the art for acid-epoxypressure-sensitive resin solutions containing a tertiary amine whichpossess adequate pot life which yield adequate rates of cure and whichyield cured pressure-sensitive resins with satisfactory releaseproperties from release substrates.

SUMMARY OF THE INVENTION

The above-mentioned need in the art is fulfilled by the presentinvention of acid-epoxy pressure-sensitive resin solutions containing atertiary amine catalyst which have adequate pot life. Thepressure-sensitive solutions contain:

A. an organic solvent solution of an interpolymer comprising:

1. between 0.1 and 15 weight per cent of an α,β-ethylenicallyunsaturated aliphatic carboxylic acid,

2. between 0.1 and 2 weight per cent of a glycidyl monomer selected fromthe group consisting of glycidyl acrylate, glycidyl methacrylate andallyl glycidyl ether,

3. between 35 to 84.9 weight per cent of a monomer selected from thegroup consisting of esters of acrylic acid and methacrylic acidcontaining from 6 to 20 carbon atoms, and

4. optionally a monomer selected from the group consisting of α-olefinscontaining 2 to 10 carbon atoms, vinyl esters of alkanoic acidscontaining from 3 to 10 carbon atoms, ethyl and methyl esters of acrylicand methacrylic acids, acrylonitrile, methacrylonitrile, styrene, andvinyl chloride, wherein the interpolymer has a weight average molecularweight in the range of 10,000 to 500,000 and a glass transitiontemperature in the range of -15° to -75°C.; and

B. 1,3-bis(dimethylamino)-2-hydroxypropane; wherein there are between0.01 and 1.0 parts by weight of the1,3-bis(dimethylamino)-2-hydroxypropane per 100 parts by weight ofinterpolymer.

The invention further provides articles of manufacture comprising filmsof pressure-sensitive resin containing the above-described interpolymerand 1,3-bis(dimethylamino)-2-hydroxypropane and provides a process forthe manufacture of such articles.

DESCRIPTION OF THE INVENTION

The practice of the present invention involves the preparation of theinterpolymer which is then formulated with the1,3-bis(dimethylamino)-2-hydroxypropane. The solution is cast on asubstrate and the resulting film is dried and cured to a permanentlytacky composition with improved cohesive strength.

The interpolymer comprises monomers selected from the groups (1), (2),(3) and (4) (described above). Group (1) monomers are exemplified byacrylic acid, methacrylic acid, crotonic acid, isocrotonic acid and thelike, maleic acid, fumaric acid, citraconic acid, itaconic acid, and thelike, and the alkyl monoesters of maleic acid, fumaric acid, citraconicacid and itaconic acid in which the alkyl group contains from 1 to 4carbon atoms such as methyl, ethyl, propyl and butyl maleates and thelike. Preferred acid monomers include acrylic acid and methacrylic acid.

Group (2) monomers are selected from the group consisting of glycidylacrylate, glycidyl methacrylate and allyl glycidyl ether.

Group (3) monomers include the esters of acrylic and methacrylic acidcontaining from 6 to 20 carbon atoms. Preferred esters contain branchedchain alkyl groups such as isobutyl acrylate, 2-ethylhexyl acrylate and2-ethylhexyl methacrylate. The pressure-sensitive resin need onlycontain monomers from Groups (1), (2), and (3). However, optionallymonomers from Group (4) may also be present. Group (4) includesα-olefins containing from 2 to 10 carbon atoms, vinyl esters of alkanoicacids containing 3 to 10 carbon atoms, such as vinyl acetate and vinyloctoate, ethyl and methyl esters of acrylic acid and methacrylic acids,acrylonitrile, methacrylonitrile, styrene and vinyl chloride.

The ratio of monomers in the interpolymer is selected so that the glasstransition temperature is in the range of -15° to -75°C. A suitableratio is conventionally calculated from the equation: ##EQU1## where Tgis the glass transition temperature of the interpolymer expressed asdegree Kelvin, Tg₁, Tg₂, etc. are the glass transition temperatures ofthe homopolymers of the respective comonomers and W₁, W₂, etc. are theweight fractions of comonomers required for a specific glass transitiontemperature of the interpolymer. Glass transition temperatures may bedetermined experimentally by conventional methods such as by means ofthe duPont Differential Thermal Analyzer.

The weight average molecular weight of the interpolymer is in the rangeof 10,000 to 500,000, corresponding to a relative viscosity in the rangeof 1.1 to 7.0 measured on a 2 weight per cent solution of interpolymerin benzene. The preferred molecular weight range is from 20,000 to300,000, providing adequate cohesive strength to the interpolymerwithout excessive solution viscosity.

The interpolymers are conveniently prepared by the process set forth inU.S. Pat. No. 3,284,423 to Edmund C. Knapp.

The solids contents of the resin solutions of this invention shouldgenerally not exceed 50% by weight if the solution is to haveappropriate viscosity and pot life. In these respects, the preferredsolutions contain ethyl acetate-hexane in the ratio of between 5:1 and3:2 and have a total solids content of about 30 to 45 per cent by weightand a viscosity within the range of about 100 to about 30,000centipoises at 25°C. Furthermore, these resin solutions are compatiblewith many common organic solvents. For example, they may be diluted withequal volumes of the following solvents to give clear, useful solutions:acetone, isopropanol, butyl alcohol, 2-ethoxyethyl alcohol,2-butoxyethyl alcohol, carbon tetrachloride, ethyl acetate, hexane,methyl alcohol, methyl isobutyl ketone, toluene, 1,1,1,-trichloroethane,and so on.

The method of polymerization is not critical in that the process can becarried out in solution or in emulsion. Care should be taken, however,to keep the reaction temperature below the level at which the acidcompound and the epoxide interact and cause gelation. The polymerizationis preferably carried out by heating the monomers at 65° to 80°C. in thepresence 0.1 to 0.5% of free radical initiator, based on the weight ofthe monomer charge.

The following resins were made by the process described in U.S. Pat. No.3,284,423. All parts and percentages are by weight unless otherwisespecified. The terms interpolymer and resin are used interchangeably.

    ______________________________________                                        EXAMPLES 1-4                                                                  Resin Solution Compositions, Parts by Weight                                             Ex.    Ex.      Ex.      Ex.                                                  1      2        3        4                                         ______________________________________                                        Acrylic Acid 6.6      6.9      7.4    5.0                                     Glycidyl Methacrylate                                                                      0.07     0.2      0.35   --                                      Allyl Glycidyl Ether                                                                       --       --       --     1.0                                     2-Ethylhexyl Acrylate                                                                      60.0     60.0     60.0   74.0                                    Methyl Acrylate                                                                            33.0     32.5     32.0   --                                      Vinyl Acetate                                                                              --       --       --     20                                      Ethyl Acetate                                                                              95.0     95.0     95.0   80.0                                    Hexane       19.0     19.0     19.0   --                                      Toluene      --       --       --     24.0                                    ______________________________________                                    

Formulation of the acid-epoxy pressure-sensitive resin solutions withthe amine catalyst is conveniently carried out by preparing a solutionof the amine in an organic solvent at a concentration between 5 and 50%and adding the solution to the pressure-sensitive resin solution slowlywith stirring to disperse the amine uniformly throughout. The amount of1,3-bis(dimethylamino)-2-hydroxypropane added to the resin solution isbetween 0.01 and 1.0 parts by weight per 100 parts by weight of resinand is preferably between 0.05 and 0.4 parts by weight for adequateviscosity stability and curing rate.

The formulated solutions are subjected to determination of viscosityover a period of at least 24 hours at 25°C. to determine the pot life.The viscosity is determined at 25°C. by the conventional method using aBrookfield Model LVF Viscometer and the appropriate spindle and speed.

Data for Resin Examples 1 and 2 diluted to 30 per cent solids arepresented in Table 1. In the Tables which follow, triethylenediamine isdesignated as TDA and 1,3-bis(dimethylamino)-2-hydroxypropane isdesignated as BDMAHP.

                  TABLE 1                                                         ______________________________________                                                             Brookfield                                                        Parts Catalyst                                                                            Viscosity                                                         per 100     (centipoises)                                            Resin Ex. 1                                                                              Parts Resin   Initial   315 Hrs.                                   ______________________________________                                        Uncatalyzed                                                                              --            3,150      3,350                                     TDA        0.2           3,550     >50,000                                               0.15          3,750      3,800                                     BDMAHP                                                                                   0.30          4,740      5,000                                     Resin Ex. 2                                                                   Uncatalyzed                                                                              --            2,300      2,400                                     TDA        0.2           2,300     Gel                                        BDMAHP     0.2           2,300      2,600                                     ______________________________________                                    

The data show that resin solutions containing1,3-bis(dimethylamino)-2-hydroxypropane are vastly superior to resinsolutions containing triethylenediamine in viscosity stability and potlife.

The effect of catalyst on the curing rate is determined by comparing thecreep resistance of the catalyzed and uncatalyzed pressure-sensitiveresins and by comparing the degree of cure of the catalyzed anduncatalyzed resins.

CREEP RESISTANCE

The pressure-sensitive resin solutions are cast on silicone releasepaper and the cast films are dried for 15 minutes at room temperatureand 2 minutes at 90°F. The films (0.8 mil thickness) are cooled to roomtemperature and transferred to a film of polyester sold under thetrademark MYLAR. The MYLAR film is cut into one-half inch strips whichare applied to polished stainless steel to form 0.25 square inch bonds.The bonds are held in a vertical plane at 70°F. and 50% relativehumidity and are loaded with one-pound weights. The time in hours forfailure of the bond after application of the load is determined. Datafor the resin of Example 1 cured for 2 minutes and for 60 minutes at100°C. are presented in Table 2.

                  TABLE 2                                                         ______________________________________                                                  Parts Catalyst                                                                            Creep Time to                                                     per 100     Failure, Hrs.                                           Resin Ex. 1 Parts Resin   2 min.   60 min.                                    ______________________________________                                        Uncatalyzed --            1.0      13                                         TDA         0.2           1.5      22                                                     0.15          1.3      29                                         BDMAHP                                                                                    0.30          1.6      41                                         ______________________________________                                    

The data show that cured films containing1,3-bis(dimethylamino)-2-hydroxypropane as the cure catalyst aresuperior in cohesive strength to films of the same polymer withoutcatalyst or with triethylenediamine catalyst.

DEGREE OF CURE

The degree of cure of the pressure-sensitive resin is determined byheating a dry film of the resin for 5 minutes at 110°C. A 1 gram sampleof the resin is stirred in a 100 ml. graduated glass cylinder with 99ml. toluene. The resin swells and dissolves at least partially. After 24hours of stirring, the swollen resin is allowed to settle in thegraduate for 24 hours. The volume of swollen resin is noted and theswelling index is calculated as the ratio of the volume of swollen resinto the volume of the unswollen resin. A 20 ml. sample of the supernatantsolvent is taken out through a plug of cotton into a pipette. The resincontent of the sample is determined, and, hence, the fractions ofsoluble resin and insoluble resin in the cured film are determined. InTable 3, data for resin Example 3 catalyzed with triethylamine,triethylenediamine and 1,3-bis(dimethylamino)-2-hydroxypropane arecompared with an uncatalyzed sample of resin. The tertiary aminecatalyst is present in a concentration of 1.78 m. moles per 100 gramsresin.

                  TABLE 3                                                         ______________________________________                                                                       Swelling                                       Resin Example 3                                                                              Insoluble Fraction                                                                            Index                                          ______________________________________                                        Uncatalyzed    0               --                                             Triethylamine  0               --                                             TDA            0.36            50                                             BDMAHP         0.10            80                                             ______________________________________                                    

The data show that triethylenediamine and1,3-bis(dimethylamino)-2-hydroxypropane are very effective curecatalysts in comparison with triethylamine. In general, an insolublefraction of 0.02 or greater is associated with appreciable cohesivestrength in the pressure-sensitive resin.

KEIL RELEASE VALUES

Keil release values for the pressure-sensitive resins are determined bycoating a silicone release paper with pressure-sensitive resin solutionand allowing the solution to dry for 15 minutes. at 70°F. and for 2minutes at 90°C. The dry film thickness is 2 mil. MYLAR polyester filmof 1 mil thickness is applied with the standard 4-pound roller specifiedby the Pressure Sensitive Tape Council to the pressure-sensitive film.Strips of the MYLAR release paper laminate 1 inch in width are cut. Thestrips are subjected to a T-peel test on an Instron Tensile TesterMachine, with a crosshead speed of 12 inches per minute. The peel valueor Keil release value expressed as grams per inch width is obtained. Therelease values, initial, after 24 hours at 70°C. and after 88 hours at70°C. are determined. The data for resin Example 2 are presented inTable 4. The catalyst is present in a concentration of 1.78 m. moles per100 grams resin.

                  TABLE 4                                                         ______________________________________                                                  Keil Release Value, grams                                                     per inch width                                                                             24 Hrs.     88 Hrs.                                    Catalyst    Initial    70°C.                                                                              70°C.                               ______________________________________                                        TDA         22         38          77                                         BDMAHP      19         25          29                                         ______________________________________                                    

Keil release values less than 30 grams per inch width are generallyacceptable. The data show the rapid increase in Keil release value ofthe resin containing triethylenediamine to an unacceptable value uponaccelerated aging at 70°.

The compositions of the present invention may be used as the adhesivecomponent in pressure sensitive tapes, films and foams. They adhere wellto resin surfaces such as plasticized poly(vinyl chloride) MYLAR,cellulose acetate, nylon, polyethylene and polypropylene, as well as topaper, metal and painted surfaces. They are especially useful as theadhesive component of decorative vinyl sheets and decals, conferringexcellent shrink resistance to vinyl film. Their outstanding tack,wetting and holding power may be used to advantage in transfer adhesiveapplications.

Articles of manufacture such as tapes, decals, decorative vinyl sheetsand transfer films containing the pressure-sensitive resin compositionof the present invention are prepared by coating the resin on theappropriate substrate by conventional coating methods. Such articlesconventionally include a release paper for temporary protection of theadhesive film until the adhesive bond is made. The thickness of theadhesive film is generally in the range of 0.2 to 5 mil.

Application of the film to the substrate is conventionally carried outon roll coaters such as reverse roll and gravure roll coaters. The resinsolution viscosity is adjusted to between 25 and 5,000 centipoises withhigher viscosities within the range preferred for reverse roll coatingand lower viscosities within the range preferred for gravure coating.The coatings are applied at a rate of between 10 feet per minute and1,000 feet per minute.

While the present invention has been described with particular referenceto certain specific embodiments thereof, it will be understood thatcertain changes, substitutions and modifications may be made thereinwithout departing from the scope thereof. This invention alsocontemplates the use of fillers, extenders, stabilizers, antioxidants,plasticizers, tackifiers, flow control agents, adhesion promoters, dyes,etc., in the pressure-sensitive resin solutions and thepressure-sensitive resins of this invention.

What is claimed is:
 1. An article of manufacture which comprises asubstrate coated on at least one side with a pressure-sensitivepermanently tacky composition comprising a dried mixture of aninterpolymer and between 0.01 and 1.0 parts by weight of1,3-bis(dimethylamino)-2-hydroxypropane per 100 parts by weight of theinterpolymer, wherein the interpolymer has a weight average molecularweight in the range of 10,000 to 500,000, a glass transition temperaturein the range of -15° to -75°C. and comprises:1. between 0.1 and 15weight percent of an α,β-ethylenically unsaturated aliphatic carboxylicacid,
 2. between 0.1 and 2 weight percent of a glycidyl monomer selectedfrom the group consisting of glycidyl acrylate, glycidyl methacrylate,and allyl glycidyl ether,
 3. between 35 to 84.9 weight percent of amonomer selected from the group consisting of esters of acrylic acid andmethacrylic acid containing from 6 to 20 carbon atoms, and 4.optionally, a monomer selected from the group consisting of α-olefinscontaining 2 to 10 carbon atoms, vinyl esters of alkanoic acidscontaining from 3 to 10 carbon atoms, vinyl and methyl esters of acrylicand methacrylic acids, acrylonitrile, methacrylonitrile, styrene andvinyl chloride.
 2. The article of manufacture of claim 1 wherein thesubstrate is a tape.
 3. The article of manufacture of claim 1 whereinthe substrate is a sheet.
 4. The article of manufacture of claim 1wherein the substrate is a sheet of silicone coated release paper. 5.The article of manufacture of claim 1 wherein the substrate comprises apolymer resin selected from the group consisting of plasticizedpoly(vinyl chloride), polyester, cellulose acetate, nylon, polyethyleneand polypropylene.